Microcontroller system having security circuitry to selectively lock portions of a program memory address space

ABSTRACT

A microcontroller system includes a security lock circuit to regulate access requests to contents of locations of a program memory. The regulation is selective, based on an operating mode of the security lock circuit, and also based on the source of the access request and the location of the program memory for which the access request is intended. Among other advantages, one major advantage provided by the security lock circuit is that concurrent programming (i.e., programming of one area of memory using instructions executing from another area of memory) can be initiated under predetermined secure conditions.

TECHNICAL FIELD

The present invention relates to microcontroller systems having flash memory in a program memory address space and, in particular, to a microcontroller having security circuitry such that a program executing from one portion of the flash memory of the microcontroller can securely program or reprogram another portion of the flash memory.

BACKGROUND

Microcontroller systems-including memory are known in the art. With a conventional microcontroller system, blocks of the memory may be locked such that a locked block cannot be interrogated. This locking thus secures program code contained in the locked blocks from unauthorized access by firmware piraters and from accidental erasing and programming.

However, conventionally, neither can the locked blocks be conveniently reprogrammed or accessed by -an authorized user. It is desirable for a microcontroller system to be configurable such that blocks of memory can be locked from unauthorized interrogation or code corruption, but can also be reprogrammed and/or accessed under a predetermined secure environment.

SUMMARY

In accordance with the present invention, a microcontroller system includes a security lock circuit to regulate access requests to contents of locations of a program memory. The regulation is selective, based on an operating mode of the security-lock circuit, and also based on the source of the access request and the location of the program memory for which the access request is intended. Among other advantages, one major advantage provided by the security lock circuit is that concurrent programming (i.e., programming of one area of memory using instructions executing from another area of memory) can be initiated under predetermined secure conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block functional illustration of an embodiment of a microcontroller system.

FIG. 2 is a block functional illustration of an embodiment of a flash operation controller of the FIG. 1 microcontroller system.

FIGS. 3A to 3C illustrate various states of program memory organization of the FIG. 1 microcontroller system.

DETAILED DESCRIPTION

FIG. 1 illustrates, in block functional form, a microcontroller system 100 in accordance with an embodiment of the present invention, unitarily formed in an integrated circuit. The system 100 includes a microcontroller unit (MCU) core 103, designated by part number T8052 in FIG. 1. In the described (preferred) embodiment, the microcontroller system 100 is compatible with standard 8xC58 microcontroller devices.

Embedded program memory 102 includes two independent program memory blocks 102 a and 102 b. The program memory blocks 102 a and 102 b are erasable and reprogrammable, and are preferably flash EEPROM of the type disclosed, for example, in U.S. Pat. No. 5,572,054, issued Nov. 5, 1996 and assigned to Silicon Storage Technology, Inc. of Sunnyvale, Calif., the assignee of the present patent application. Program memory block 102 a (designated, for convenience of reference, as the “lower program block”) is addressable at the standard 87C58 32 Kbytes of internal ROM area, and program memory block 102 b (designated for convenience of reference as the “upper program memory block”) is, addressable at the high address end of the 8xC5x architecture. In addition, the microcontroller system 100 is configurable to address up to 64 Kbytes of program memory (not shown) external to the system 100. The system 100 also includes on-chip RAM (not shown) and is configurable to address external data RAM (not shown).

A flash operation controller 104 (shown in more detail in FIG. 2, discussed later) includes circuitry for controlling access to and from the program flash memory 102. A command/address decoder 106 interfaces the MCU core 103 to the addressable memory (whether program memory 102 or other addressable memory, internal or external), I/O ports, and configuration registers. Some of the configuration registers, particularly those within the flash operation controller, are discussed in greater detail later. When the system 100 is operating with the MCU core 103 (as opposed to an external processor, discussed below), this is known as “normal mode”.

I/O pads interface. 108 includes circuitry for interfacing the various components of the system 100 to external devices (not shown), for example, external memory or an external host. Finally, test mode controller 110 assists in controlling the system 100 when it is operating in “external host mode”, wherein an external “host” processor (not shown) is operating in place of the MCU core 103. An example of an external host is a PCB tester automated test equipment, a PC-controlled development board, or an OTP (one-time programmable) MCU programmer. During external host mode, in response to hardware commands decoded from control signal input pins, the system 100 performs test, erase and/or programming functions.

Security functions of the microcontroller system 100 are now discussed with reference to FIG. 2, which is a more detailed functional diagram of the flash operation controller 104. In the context of the present invention, a primary function of the flash operation controller 104 is to control read and/or write access to the blocks 102 a and 102 b of the program memory 102, in accordance with the value stored in a “security byte”. Specifically, the flash control unit 201 controls such access responsive to the value of the security byte, as indicated by a field of the SuperFlash® Configuration register (SFCF) 202 which contains a decoding of the security byte. The security byte is located at the highest address location, FFFFh, of the microcontroller system 100 memory space. That is, the security byte is located at the highest location in upper program memory block 102 b.

Depending on the value of the security byte, either the entire program memory 102 may be locked; just the upper program memory block 102 b may be locked; or both the lower program memory block 102 a and the upper program memory block 102 b may be unlocked. (Again, the term “value of the security byte” is a shorthand notation herein for the value of the field in the SFCF register 202 that is a decoding of the security byte.) Precisely what it means for a block of memory to be “locked” or “unlocked” is discussed later. Table 1 shown below provides a summary of the effect that a particular value of the security byte has on the security functions of the system 100.

TABLE 1 Sec SFCF Blk Block Sector Write Burst Verify Byte (6:5) EA# Sel Erase Erase Byte Write Byte Description FF 00 X X Y Y Y Y Y No lock 55 11 X X N N N N N Both locked F5 01 X 0 Y Y Y Y Y Only block 1 (4KB) is X 1 N N N N N locked 05 10 X 1 N N N N N Both locked but block 0 is 0 0 N N N N N programmable by software 1 0 Y Y Y Y Y code resided in block 1 Note: SecByte = Value of Security Byte at location FFFFh; SFCF(6:5) = Bit 5 and 6 of SFCF register; EA# = Ext. Access enable input pin: 1 - running code from internal memory, 0 - running code from external enable; blkSel = Block Select signal (internal): 1 - block 1 (4Kx8), 0 - block 0 (32Kx8); X = don't care; Y = command allowed; N = command not allowed.

When the security byte has a value FFh, both the program memory block 102 a and program memory block 102 b are unlocked. When the system 100 is delivered from the factory, or after a CHIP ERASE function whereby the contents of the program memory 102 are erased and there is no need for security, the default value of the security byte is FFh.

Referring also to Table 2 shown below, if the security byte value is other than FFh (i.e., has an “activated” value), then security is activated to lock at least one of the program memory blocks 102 a and 102 b. In this case, the “move constant” (MOVC) instruction, which in some instances could be used for read access of data from the program memory 102, is denied read access when the location of the MOVC instruction itself could be in either an unlocked flash memory location (i.e., a portion of the flash memory 102 is unlocked, as indicated by the activated value of the security byte) or may be in an external memory (as indicated by the signal (D at an External Enable, or EA#, input pin). Therefore, the contents of the internal flash program memory 102 are prevented from being read undesirably when any level of security is activated.

TABLE 2 Internal and External Program Memory Access with Security Lock activated (the value of the security byte is neither 00h nor FFh). ACCESS TO LOCKED ACCESS TO UNLOCKED OR PROGRAM EXTERNAL PROGRAM INSTRUCTIONS MEMORY MEMORY MOVC in locked YES YES program memory MOVC in unlocked or NO YES external program memory

The effect of the security byte on program memory 102 programming operations (i.e., write access) is now discussed. If the system 100 is in external host mode and the security byte. value is other than FFh, the following external host mode commands are not allowed on the locked program memory 102:

sector erase, to erase the addressed sector;

block erase, to erase the addressed block;

write byte, to write a byte to the addressed location;

burst write, to write an entire addressed row by burst writing sequentially within the row if the byte location has been successfully erased and not yet programmed; and

verify byte, to read a code byte from the addressed location.

The read ID function, to read the chip “signature” and identification registers, is allowed even when the security byte is not FFh.

Put simply, .when the system 100 is in external host mode, only the CHIP ERASE operation, to completely erase the program memory 102, can be used to access the lockable flash memory locations. That is, the CHIP ERASE function operates regardless of the value, of the security byte. As discussed above, in response to the CHIP ERASE function being executed, the security byte is set back to the default value of FFh. Then, because security is disabled, the external host may freely program (or otherwise access) the program memory 102, via the SFDT “mailbox register” 204. (The WDTD and WDTC registers shown in FIG. 2 are for controlling a watchdog timer and are not relevant to the present disclosure.)

If the security byte at location FFFFh has the value F5h, only the upper program memory block 102 b is locked, and the lower program memory block 102 a is not locked.

During concurrent programming mode, with the security byte at location FFFFh set to 05h, the lower program memory block 102 a may be programmed (i.e., sector or block erase, write and verify operations) by program code executing from the upper program memory block 102 b, but not by any program code executing from the external program code storage even though both program memory blocks 102 a and 102 b are security locked for any other access, such as External Mode Host Access. This security mechanism is known as “soft lock”. Contrary to the “hard lock”, when the security byte at location FFFFh is set to 55h (where both the upper 102 b and lower 102 a memory blocks are not accessible for any programming operation), the “soft lock” security mechanism allows the system to use a secure program code to perform in-system and in-field flash memory concurrent programming of the lower program memory block 102 a under a predetermined secure environment. Table 1 further illustrates this security lock feature.

More details of the program memory addressable space are now discussed with reference to FIGS. 3A to 3C, particularly relative to how the program memory organization is adaptable to address safety and security concerns. More specifically, FIGS. 3A to 3C show three program memory organization and assignment options of the microcontroller system 100.

As discussed above, the program (flash) memory 102 of the microcontroller system 100 includes two blocks, lower program memory block 102 a and upper program memory block 102 b.

In one embodiment, the lower program memory block 102 a has 32 Kbytes and occupies the address space 0000h to 7FFFh. The 32 Kbytes of the lower program memory block 102 a are organized as 512 rows (256 of 128-byte sectors per page, 9 X-address lines) and 512 columns (64 bytes per row, 6 Y-address lines). The upper program memory block 102 b has 4 Kbytes, and occupies the address space F000h to FFFFh. The 4 Kbytes of the upper program memory block 102 b are organized as 128 rows (64 of 64-byte sectors per page, 7 X-address lines) and 256 columns (32 bytes per row, 5 Y-address lines).

As can be seen from FIGS. 3A to 3C, the address space 8000h to EFFFh is always reserved for external memory (not shown), which may be, for example, memory of a development system or hardware programmer.

Referring now specifically to FIG. 3B, in “normal mode”, when instructions are being executed by the MCU processor core 103, the upper program memory block 102 b is “hidden for code fetching”. This is so even when internal code execution is enabled (#EA=1; see Table 1) such that code may be executed from the internal program memory 102. By “hidden for code fetching”, it is meant that program instructions in the block “hidden for code fetching” cannot be fetched and executed from locations of the hidden block.

As shown in FIG. 3A, to make the upper program memory 102 b visible for code fetching, so that the concurrent programming operation can be carried out by executing program instructions from the upper program memory 102 b, the system 100 must be in internal mode (i.e., #EA=1), and the user must set a concurrent programming field in the SuperFlash® configuration register SFCF 202. In one embodiment, the concurrent programming field is the most significant bit of the SuperFlash® configuration register SFCF 202 (FIG. 2).

As for the visibility of the lower program memory block 102 a, while internal code execution is enabled (i.e., #EA=1), the lower program memory block 102 a is always visible for code fetching. This can be seen from FIGS. 3A and 3B. If the external enable is asserted at hardware reset of the system 100, program execution starts from the lower program memory block 102 a.

Finally, as can be seen from FIG. 3C, while internal code execution is not enabled (i.e., #EA=0), both lower program memory block 102 a and upper program memory block 102 b are hidden for code fetching.

By having circuitry to so allocate the program memory, the security of the program memory is enhanced. Furthermore, in combination with the security lock circuitry, the security of the program is further enhanced. 

What is claimed is:
 1. A method of operating a microcontroller system in one of a plurality of modes of operation, said system having a microcontroller and a first programmable memory block storing a first program, and a second programmable memory block storing a second program said method of operating said one mode comprising: reading said first program from of said first programmable memory block by said microcontroller; reading said second program from said second programmable memory block; and reprogramming said first program solely by executing said second program.
 2. The method of claim 1 wherein said microcontroller further blocks writing into said first and second programmable memory blocks by signals external to said microcontroller system while reading said first program and reading and executing said second program.
 3. The method of claim 2 wherein said microcontroller system further comprising a security lock circuit and wherein said microcontroller system operates in response to said security lock circuit.
 4. The method of claim 1 wherein said first programmable memory block and said second programmable memory block are portions of a flash memory.
 5. A unitary integrated microcontroller system operable in one of a plurality of modes comprising: a microcontroller; a first programmable memory block for storing a first program; a second programmable memory block for storing a second program; a storage for storing a security value; and a security circuit responsive to said security value for operating in said one mode, causing said microcontroller to read said first program from said first programmable memory block, read said second program from said second programmable memory block, and reprogramming said first program solely by executing said second program.
 6. The system of claim 5 wherein said security circuit responsive to said security value further causes said first and second programmable memory blocks from being written by signals external to said microcontroller system.
 7. The system of claim 6 wherein said first programmable memory block and said second programmable memory block are portions of a flash memory. 